Neutral Grounding Transformer (NGT) Calculator

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Neutral Grounding Transformer (NGT) Calculator

When a transformer is used to provide a neutral point for grounding in a system where the neutral point of the three phase system is unavailable or the transformers or generators are delta connected, the transformer is referred to as neutral grounding transformer (NGT).

The NG cubicle is used to install a Neutral Grounding Transformer (NGT) and a Neutral Grounding Resister (NGR) for grounding/earthing neutral current during an unbalanced three phase load of a generator, which is primarily necessary for protecting the generator winding.

The following overview describes the generator’s power, capacitance, resistance, & Report Word current ratings to provide proper rating and protection for operational conditions.

The generator has high power and voltage ratings. It has an intrinsic capacitance per phase, and extra linked capacitances adds to the overall capacitance per phase. The total capacitance is computed by adding an additional capacitance component to each phase.

The total capacitance throughout all phases generates a given capacitive reactance, which leads to a specific capacitor current. A computed tolerance is determined after considering field forcing.

The Next Generation Transformer (NGT) has a specific voltage rating and insulation level. The power rating of the NGT is derived by multiplying the field forcing allowed by the voltage rating, and the result is rounded to the nearest standard rating.

The corresponding primary resistance has been established. The secondary resistance is calculated based on the secondary voltage selected. After calculating for the transformer’s resistance, the external resistance is calculated.

The current rating is computed using the power rating and external resistance to yield a specific current value.

  • Next Generation Transformer (NGT) Rating: Rated for a specific power level for a limited time.
  • Neutral Grounding Resistors (NGR) Rating: Rated with a specified resistance value and tolerance.

To calculate the ratings of a Neutral Grounding Transformer (NGT), apply the formulas listed below:

NGT kVA=Field Forcing Allowance X NGT Voltage (kV)

Step-1: Set the primary resistance to the predicted capacitive reactance.

Step-2: To calculate the secondary resistance, multiply the primary resistance by the square of the secondary voltage-to-primary voltage ratio.

Step-3: Subtract the transformer resistance from the secondary resistance to calculate the external resistance.

Step-4: To get the current rating, divide the NGT kVA rating by the external resistance & take the square root.

Step-5: To get the NGT kVA rating, multiply the field forcing allowed by the NGT voltage.

These formulas and calculation steps provide a complete technique for computing the parameters required for a Neutral Grounding Transformer.

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